ET 332b Ac Motors, Generators and Power Systems lesson14_et332b.pptx 1 Learning Objectives After this presentation you will be able to: List the characteristics of NEMA Design motors. Identify and interpret motor nameplate data. Use kvacodes to compute motor starting currents. Explain the effects of reduced motor voltage and frequency on machine performance. Compute the changes in motor speed and torque when voltage and frequency change. lesson14_et332b.pptx 2 1
NEMA Motor Designs Different motor conductor designs given different rotor resistances, which gives different motor characteristics Design A - Starting torque 150% rated; breakdown torque 275% Starting current 7-10 times rated current Design B - Starting torque approx. 150% rated; breakdown torque 225%; starting current < 6.4 times rated. Most commonly used motor design lesson14_et332b.pptx 3 NEMA Motor Designs Design C - High starting torque motors; starting torque 240% to 275% of rated; starting I < 6.4 time I rated Design D - High slip motors. Very high starting torques 275-300% of rated. Speed operates at 85-95% rated; slip 5-15%. Used to start high inertial loads Very low starting current, near rated at start. lesson14_et332b.pptx 4 2
Motor Nameplate Data Nominal Efficiency - minimum efficiency that is guaranteed for design class. Given for one value of output -rated Design Letter - indicates NEMA design type A, B, C, D Service Factor - (S.F.) number that indicates the maximum permissible loading lesson14_et332b.pptx 5 Motor Nameplate Data Insulation Class -specifies maximum allowable temperature rise for motor windings Code Letter - means of determining the expected locked-rotor inrush current at rated voltage and rated frequency lesson14_et332b.pptx 6 3
Motor Nameplate Data Code Letter kva Table Code Letter kva/hp Code Letter kva/hp A 0-3.15 K 8.0-9.0 B 3.15-3.55 L 9.0-10.0 C 3.55-4.0 M 10-11.2 D 4.0-4.5 N 11.2-12.5 E 4.5-5.0 P 12.5-14.0 F 5.0-5.6 R 14.0-16.0 G 5.6-6.3 S 16.0-18.0 H 6.3-7.1 T 18.0-20.0 Above can be used to compute the range of starting currents J 7.1-8.0 U 20-22.4 V >22.4 lesson14_et332b.pptx 7 Starting Current kva Codes Use kva codes to find the range of motor starting currents Where: V LL = line-to-line voltage applied to motor terminals I L = line value of starting current. hp = rated motor horsepower kva comes from given table Example: G = 5.6-6.3 kva/hp lesson14_et332b.pptx 8 4
Starting Current kva Codes Example 14-1: A NEMA design motor is rated at 150 hp at 460, 60 Hz. It has a rated current of 163 A and a nominal efficiency of 96.2%. The locked rotor code is G. Find the range of starting current that can be expected from this machine. Range for code G 5.6-6.3 kva/hp Low value of kva High value of kva Low value of starting current High value of starting current 840 kva 1000 I lr := 3 460 V I lr = 1054.3A A Range 1054.3 I lr 1186.1 lesson14_et332b.pptx 9 Effects of Reduced Terminal Voltage and Frequency Machines can operate +- 10% of rated terminal voltage without significant change in characteristics. Motor rated voltage 460 V, Supply voltage 480 V Vsupply %V= 100% V rated 480 V %V= 100% 460 V %V= 104% Generally must change both voltage and frequency to maintain torque-speed characteristic (constant flux) (Can't vary terminal voltage for speed control) lesson14_et332b.pptx 10 5
Effects of Changing Voltage and Frequency on Torque Use the following empirical formula T D 2 V s = k f s 0.03 Where: T D = motor developed torque V = motor terminal voltage f = motor operating f s = per unit slip k = proportionality constant Torque proportional to V 2, s and inversely proportional to f lesson14_et332b.pptx 11 Example 14-2: A 3-phase 460 V, 20 HP, 60 Hz, 4 pole motor drives a constant torque load at rated shaft power at rated voltage, and frequency. The motor speed under these conditions is 1762 RPM. A system disturbance lowers the motor voltage by 10% and the system frequency by 6%. Find: a.) the new motor speed; b.) the new shaft power. Assume that the mechanical losses (P fw and P stray ) are constant. lesson14_et332b.pptx 12 6
Example 14-2 Solution (1) Define equations For 10% voltage reduction Find s 1. This requires synchronous speed lesson14_et332b.pptx 13 Example 14-2 Solution (2) Compute slip for 1 st case Constant torque load so equate torques Solve for s 2 lesson14_et332b.pptx 14 7
Example 14-2 Solution (3) Compute the value of s 2 Per Unit produces the same result lesson14_et332b.pptx 15 Example 14-2 Solution (4) Need synchronous speed for second case Now compute the new motor speed lesson14_et332b.pptx 16 8
Example 14-2 Solution (5) Solve part b. P 1 =rated horsepower=20 hp T s1 = shaft torque state 1 T s2 = shaft torque state 2 For constant torque load T s1 =T s2 lesson14_et332b.pptx 17 ET 332b Ac Motors, Generators and Power Systems lesson14_et332b.pptx 18 9